Gyrodynes or gyroscopic actuators (also known as “control moment gyros” (CMG)) have a wheel or “spinner” mounted on a support referred to as “gimbals” and steerable on the platform of the satellite by a motor to turn about at least one axis that is orthogonal to the axis of rotation of the spinner. An example of a gyrodyne is described in French patent application No. 98/00556, to which reference can be made.
In order to make it possible to bring a three-dimensional frame of reference associated with the satellite into any attitude a cluster of gyrodynes comprises at least three gyrodynes, or for control about two axes it comprises at least two gyrodynes. In general, at least four gyrodynes are used to provide redundancy.
The gyrodyne cluster constitutes an inertial actuator which can be controlled to apply a torque imparting an angular speed profile to the satellite platform as specified by uploading from the ground or by performing calculations on board.
In order to control the attitude, a control system determines the torque to be applied and deduces therefrom a speed that should be applied to the gimbals of the gyrodynes. The ability to deliver a total torque C is consequently not steady and non-linear. It can be written in matrix form:C=A(σ).{dot over (σ)}  (1)where A is the Jacobean matrix aij=∂Hi/∂σj, with i=1 to 3 and j=1 to 4 (or more generally taking all values from 1 to the number of gyrodynes). Starting from the torque C to be delivered, a conventional steering method consists in inverting equation (1) to obtain the reference speeds c to be given to the gyrodynes. The principle of such control is given, for example, in the above-mentioned French patent application or in U.S. Pat. No. 6,131,056.
French patent application No. 98/14548, to which reference may be made, describes and claims a method of controlling the attitude of a satellite enabling the problem of singularities to be avoided, i.e. avoiding entering into configurations in which it is impossible to obtain torque in a determined direction, by controlling a direct passage from an initial configuration to a final configuration.
For this purpose, the architecture described in patent application Ser. No. 98/14548 comprises a cluster of at least four gyrodynes having respective spinners mounted on respective gimbals, themselves mounted to turn relative to the satellite platform about gimbal orientation axes that are all different.
That configuration presents the advantage in principle of having maximum isotropy when the four gimbal axes are directed along the diagonals of a cube. When it is desired to increase capacity along two orthogonal axes in order to take account of the fact that agility requirements are rarely isotropic, it suffices to flatten the pyramid whose edges are orthogonal to the axes of the gimbals. However, as a counterpart to those advantages, the pyramid-shaped architecture presents various drawbacks. The capacity envelope presents concave portions: with a flattened pyramid it is not possible to independently adjust the capacities along the two axes of the pyramid base. The loss of one of the actuators leads to an agility envelope that is very asymmetrical and has a severe effect on agility.